Current normalizer for particle size analysis apparatus

ABSTRACT

A current normalizer utilizes a pair of parallel, simultaneously programmed current generators to provide a DC current and a synthetic particle current pulse to an electric particle sensing zone for measuring the resistance of an electrolyte. The current normalizer also includes means for controlling the generation of current in response to the setting of an adjustable precision voltage source.

[ CURRENT NORMALIZER FOR PARTICLE SIZE ANALYSIS APPARATUS Inventor:Geoffrey T. Haigh, Lake Parsippany,

[73] Assignee: Particle Data, Inc., Elmhurst, Ill.

July 10, 1973 3,394,303 7/ 1968 Cressman 324/37 Primary Examiner-AlfredE. Smith Attorney-Carlton Hill, J. Arthur Gross et al.

A current normalizer utilizes a pair of parallel, simultaneouslyprogrammed current generators to provide a DC current and a syntheticparticle current pulse to an electric particle sensing zone formeasuring the resistance of an electrolyte. The current normalizer alsoin- ABSTRACT cludes means for controllingthe generation of current inresponse to the setting of an adjustable precision voltage source.

10 Claims, 5 Drawing Figures r 7 9mm. MEANS 22 Filed: June 21,1971

21 Appl. No.: 154,859

52 US. Cl 324/71 CP [51 1111.01. ..G0ln 27/00 [58] Field of Search324/71 CP, 57 Pl, 324/63, 37; 343/177 [56] References Cited UNITEDSTATES PATENTS 3,259,242 7/1966 Coulter 324/71 CP NORMALIZATIONCONDITIONING APPARATUS /4 m VARIABLE /6-/ /6 F swlrcl-img/ 5 CIRCUIT13/5 VOLTAGE PULSE GENERATOR 'PAIENIE JUL 1mm SHEET 2 BF 2 INVENTUR.6620/2929 Iv v ATTORNEYS CURRENT NORMALIZER FOR PARTICLE SIZE ANALYSISAPPARATUS BACKGROUND OF THE INVENTION comprises a beaker having anelectrolyte containing t suspended particles therein and an orifice tubecontaining electrolyte immersed in the electrolyte of the beaker. Oneterminal from a current source is positioned within the orifice tube andthe other terminal of the current source is positioned within the beakerwhereby modulations of the developed voltage across the orifice in theformof particle pulses may be sensed as a sample of suspended particlesto be analyzed is caused to flow through theorifice. When a particletraverses a given orifice, there will be a change in the resistance ofthe orifice proportional to the product of the volume of the particleand the resistivity of the electrolyte. For example, for an orificewhich measures 10,000 ohms, a 10- ohm change for a particle entering agiven orifice might be obtained. If the resistivity of the electrolyteis changed such that the orifice resistance is 20,000 ohms, a 20-ohmchange will occur for the same particle. However, when a current isforced through the electrolyte, some degree of back voltage is generateddue to polarization at the electrodes.

If the current through the orifice is held constant, the momentarycurrent change caused by passage of a particle is independent ofelectrolyte resistivity. If the voltage drop across the orifice alone isheld constant, the voltage change caused by passage of a particle isindependent of electrolyte conductivity," but holding the voltageconstant is practically not achievable; whereas, holding currentconstant is practically achievable. When the amplifier input impedanceis nearly matched to that of the orifice, a condition generallydesirable for best signal to noise ratio, the amplifier is sensingpartly voltage change and partly current change at the orifice.Therefore, the sensed signal is proportional to the volume of theparticle and some fractional power of the resistivity of theelectrolyte. correspondingly, the provision of a voltage source toprogram the orifice is insufficient because the counter-emf generated byelectrode polarization is a variable.

SUMMARY OF THE INVENTION The instant invention retains the desirablecondition of near impedance matching while providing means fornormalizing the signal amplitudes of all particles over presentinvention therefore proposes a method to measure the resistive elementof the orifice and adjust the system to compensate for that resistance.

A method for achieving the foregoing objective, hereinafter callednormalization, and apparatus for carrying out the method employ a DCcurrent generator to drive theorifice, which generator is programmableby suitable means, such as a potentiometer, rotary switch, selectorswitch, etc. A second current generator is provided and issimultaneously programmable with thefirst generator and providesGaussian-shaped pulses of predetermined amplitude in the current realm.The current pulses simulate particle pulses and are injected intotheorifice with the standard DC current. With the two current generatorsbeing simultaneously programmable, if the DC current is, for example,doubled, the amplitude of the current pulse will be simultaneouslydoubled. The current normalizer is therefore provided with means forproviding an accurate DC current to the orifice including a precisionvoltage source. The normalizer is also provided with means foraccurately generating current pulses (synthetic particle pulses), thislatter current means also being controlled by the precision voltagesource. Inasmuch as the orifice circuit is usually provided with a largefiltering capacitor and some modestly large value of resistance, and asthe normalizing pulses would therefore be ineffective, the normalizer isprovided with means for bypassing this filter network so that thenormalizing pulses are applied directly to the orifice electrodes. Inaddition, since the orifice circuit normally has associated therewith anamplifier for sensing particle pulses, this amplifier is provided with atake-off point which has positive unity gain with respect to the inputsignal. The resultant signal is fed back to the orifice couplingresistors such that a very high impedance is synthesized at saidresistors in order to reduce the loading effect of said resistors.

BRIEF DESCRIPTION OF THE DRAWINGS Many other objects, features andadvantages of the invention, its organization, construction andoperation, will best be understood from the following detaileddescription of an exemplary embodiment thereof taken in conjunction withthe accompanying drawings, in which:

FIG. 1 is a diagram which explains the electrical characteristics of anorifice;

FIG. 2 is an electrical circuit diagram of an orifice having a sensingDC current supplied thereto;

FIG. 3 is a schematic circuit diagram illustrating current normalizationin accordance with the principles of the present invention; i

FIG. 4 is a schematic block diagram of a current normalizer constructedin accordance with the principles of the present invention; and

FIG. 5 is a schematic circuit diagram of the current normalizerillustrated in block form in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT GENERAL DESCRIPTION Referring toFIG. 1, an orifice circuit is schematically illustrated at l ascomprising a pair of electrodes 2, 3 on each side of an orifice 4. Thisstructure is the equivalent of a series circuit including a firstresistor 5 which represents the electrical resistance R of the orifice,a resistor 6 which represents the change AR of the resistance R andwhich is proportional to the resistance R times the voltage Ae developedacross the resistance due to a particle traversing the orifice 4, and abattery 7 which represents the counter emf of the contact potentials ofthe electrodes 2, 3.

FIG. 2 illustrates a conventional orifice circuit wherein a DC source 8is connected to the orifice circuit to provide a current i, to theorifice circuit, this current supply being bypassed for AC noise by acapacitor 8a. The voltage pulse generated in response to a particletraversing the orifice is therefore approximately equal to the currenti, times the change in orifice resistance AR which is the current i,times the voltage Ae times the resistance R. As previously stated, inorder that a particle measuring system of the electric sensing zone typemay be calibrated under all conditions, it is necessary that a particleof given size in a given orifice provide the same response and thus itis necessary to measure the resistance R of the orifice for a givenelectrolyte.

Referring to FIG. 3, the orifice circuit 5, 6, 7 is provided with afirst current generator 8 and a second current generator 9. The firstcurrent generator 8 is like that illustrated in FIG. 2; however, in FIG.3, this generator is shown to be programmable by a variable controlmeans 10. The second current generator provides particle pulsesimulation and is illustrated to be simultaneously variable with thegenerator 8 by means of the control means 10. The orifice circuit isconnected to an amplifier 11 for sensing the particle pulses and a pulseheight analyzer (PI-IA) 12 is provided for performing size analysis asis well known in the art. The FHA may include an oscilloscope as a meansof providing a display in response to detection of particle pulses bythe amplifier 1 1.

Referring to FIG. 4, the two current generators 8, 9 are illustrated asbeing combined in a single circuit 13 which is programmed by a controlmeans 10. The control means is provided with means 14 for conditioningthe apparatus for the normalization process. A voltage pulse generator15 is provided to supply the circuit 13 with voltage pulses to effectthe generation of current pulses and a switching circuit 16 is providedto (1 place the initiation of voltage pulses under the control of theprecision voltage supply of the control means 10 and (2) control thebypassing of the filter circuit (not shown) of the orifice circuit, aswill be discussed in greater detail below.

DETAILED DESCRIPTION Referring to FIG. 5, the apparatus of FIG. 4 isillustrated in greater detail. The orifice 1 is connected to anamplifier 11 for sensing particle pulses and provided the same to PI-IAequipment 12. The orifice 1 and the input of the amplifier 1 1 areconnected by way of a pair of resistors 19, and a switch contact 16-2,which will be dealt with below, to the programmable current source 13.

The programmable current source 13 comprises a pair of transistors 17and 18 connected in a Darlington configuration. The emitter of thetransistor 18 is connected to a suitable voltage reference point by wayof a resistor 21 and a potentiometer 22. The potentiometer 22 mayadvantageously be located on a control panel. The base of the transistor17 is held at a precision voltage level by means of an operationalamplifier 25 and its associated feedback loop which senses the voltageat the emitter of the transistor 18. The feedback loop includes aresistor 27 and a capacitor 28 in parallel therewith, the combinationbeing serially connected between the emitter of the transistor 18 andthe inverting input 26 of the amplifier 25. For the amplifier 25, anyvoltage applied to the input 30 must, by the stability criteria for anoperational amplifier, be identical to the voltage found on theinverting input 26. The inverting input 26, however, is sensing thevoltage at the emitter of the transistor 18; therefore, as the voltageis varied at the input 30, the base voltages of the transistors 17, 18shift such that the voltage at the emitter of the transistor 18 is heldat a precision reference point with respect to the reference voltage atthe point 23. Hence, the resistor 21 and the potentiometer 22 serve toform a resistive reference for the current generator at the collectorsof the transistors 17, 18.

A controlled precision voltage is provided to the input 30 of theamplifier 25 by way of the control means 10 which comprises a pair oftransistors 31, 32. The transistor 31 includes an emitter which isconnected to the collector of the transistor 32 by way of a resistor 33and on to a point between 230 volts and ground at a junction of a powersupply terminal of the operational amplifier 25 and a resistor 29. Theoperational amplifier may for example be of the 709 type wherein theseventh pin is connected to the resistor 29 and the fourth pin isconnected to 230 volts DC. The collector of the transistor 32 and theemitter of the transistor 31, by way of the resistor 33, are connectedto the reference point 23 by way of a zener diode 59 having a pair ofcapacitors 60, 61 connected in parallel therewith. In a particularrealization of this circuit, the zener diode 59 was a l2-volt device.

The emitter of the transistor 32 is connected to the collector of thetransistor 31 by means of a resistor 35 and on to the reference point 23by way of a zener diode 36 having a pair of capacitors 37, 38 connectedin parallel therewith. In the above-mentioned realization of thisparticular circuit, the zener diode 36 was also a l2-volt device. I

The emitter of the transistor 32 is furtherconnected to the referencepoint 23 by way of the divider 50 and its serially connected resistors51-56, and to the input 30 of the operational amplifier 25 by way of amovable tap 57 of the divider 50 and a resistor 58.

The base of the transistor 31 is coupled to the reference point 23 byway of a capacitor 39 and is connected to the voltage divider 41-44 byway of a resistor 40.

The voltage dividers 41-44 and 50 provide precision voltages to theinput 30 of the transistor 25. The divider 50 is variable in precision 2to 1 steps yielding doublings in current change for each step and thepotentiometer 44 is connected in parallel with the resistors 41, 42 suchthat over a 2 to 1 range, the change in voltage appearing at the input30 of the amplifier 25 due to movement of the wiper of the potentiometerapproximates, by hyperbolic approximation, the logarithm over a 2 to 1range such that if a numbered dial from zero to one were associated withthe shaft of the potentiometer, the zero to the one range of the dialcould be considered as a mantissa of the logarithm to the base 2 for thechange in voltage and, hence the change in current of the currentgenerator. There is therefore provided a continuously variable fineadjustment, logarithmic in nature, with respect to rotation of the shaftof the potentiometer 44. This paralleling technique has been provided sothat one could utilize, in lieu of a single turn log pot, a IO-tumlinear pot for obtaining a logarithmic function of rotation.

The method of generating current pulses is to superimpose on the input30 of the amplifier 25, small voltage pulses of constant fixed amplitudewhose shape closely approximates the shape of pulses that would begenerated by particles passing through the actual orifice, i.e. particlepulse simulation. The pulse generator comprises a unijunction transistor62 having its emitter electrode connected to the reference point 23 byway of a capacitor 63 and a resistor 64, and connectible to thepotential of the conductor 34 by way of a resistor 65, a resistor 66 anda switch contact 16-1. (Due base of the unijunction transistor 62 isalso connectible to the potential on the conductor 34 by way of theresistor 66 and the switch 16-1, while the other base of the transistoris connected to the reference point 23 by way of a resistor 69, acapacitor 70, and a resistor network 71-74. This base is further coupledto the input 30 of the operational amplifier by way of a capacitor 75.Upon firing of the unijunction transistor 62, small current pulses aredrawn from its power leads which would appear on the powerline bus andcause errors in the output signal; therefore, the circuit is providedwith capacitance, by means of a capacitor 67 and a capacitor 68, toeliminate these small current pulses.

Upon closure of the contact 16-1, which will be explained below, thecapacitor 63 begins to charge and the unijunction transistor 62subsequently fires, as is well known in the art. The pulses generated bythe unijunction circuit are therefore coupled to the input by way of thecapacitor 75.

Given a setting of the potentiometer 44, there will be a given voltageapplied to the input 30 of the operational amplifier 25 and for a givensetting of the resistor 22 (the normalize potentiometer) there will be agiven current generated at the junction of the resistor 19 and thecapacitor 78. In order to accomplish the normalization function, thatis, with the pulse train, it is necessary that the pulses appearing atthe junction of the resistor 19 and the capacitor 78 bear a specificpredetermined relationship to the current generated in the DC realm. If,for example, a 4 milliamp current is generated at this junction, theremust at the same time be a current pulse of, for example, 1 microarnp.Hence, it is simply a matter of providing a precision resistor betweenthe point and ground, for example, a 10,000 ohm resistor, and adjustingthe potentiometer 22 to provide precisely volts across the resistor toeffect a 4 milliamp current. At this setting, that is, with the dividerset at the 4 milliamp position, the precision resistor 74 is adjusted toyield current pulses of the desired amplitude at the junction of theresistor 19 and the capacitor 78. In this manner, all circuit boards maybe fabricated to yield the same identical results.

The current at the junction of the resistor 19 and the capacitor 78 istherefore a function of the voltage appearing at the input 30 of theoperational amplifier 25 divided by the resistance between the emitterof the transistor 18 and the reference point 23, which applies to boththe AC and DC current realms simultaneously. Therefore, adjustment ofthe potentiometer to vary the size of pulses viewed on an oscilloscopeand seen at the junction of the resistor 19 and the capacitor 78 willsimultaneously vary the DC current appearing at that point. Thetransistor 45 and its associated components together form a simpleswitch which, with the emitter of the transistor 45 held at ground or DCzero, prevents current flow through the collector of the transistor 45to in turn prevent operation of the precision controlled voltage source10. With the emitter of the transistor 45 at ground, therefore, novoltage is applied to the input 30 of the operational amplifier 25 andthe apparatus is not operable. When the ground or zero (stop signal) isremoved from the emitter of the transistor 45, the circuit isconditioned for operation. This stop signal is provided as a safetymeasure in that when the mercury column in the sensing apparatus reachesthe upper limit of its travel, it grounds the emitter of the transistor45 to turn off the entire current generator and therefore remove voltagefrom the orifice circuit, in particular from the orifice electrodes toprevent shock to persons raising and lowering samples and so forth.

The transistors 31 and 32, the zener diode 49, and the potentiometer 44together form a temperature stable precision variable voltage source.The zener diode 49' is programmed by the current from the transistor 45,when the emitter of the transistor 45 is held at +5 volts or greater inthis particular example, and a reference voltage with respectto thepoint 23 is generated and provided at the upper terminal of thepotentiometer 44. Some fraction of this voltage appears on the wiper armof the potentiometer 44 and is applied to the base of the transistor 31,an emitter follower, followed by the transistor 32, a complementaryemitter follower, which is arranged such that the base-emitter voltageand temperature coefficient of these transistors are effective to cancelthe effects of temperature changes. Therefore, the emitter of thetransistor 32 provides the divider 50 with a precision variablereference voltage and the divider provides some fraction of that voltageto the input 30 of the operational amplifier 25.

All signals on the circuit boardare referenced to the point 23, thevoltage for the remainder of the board being derived by way of adropping resistor 29, the zener diodes 36, 59 and a connection to 230,volts at the operational amplifier 25. In the particular realization ofthis circuit discussed above, the point 23 carried 2l8 volts and theconductor 34 was at 2'06 volts. These voltages serve to form the l2-voltsupplies to drive the operational amplifier 25 and the remainingtransistors.

The circuit is provided with a normalize button 16-5 connected in serieswith a winding 16-3 of a multipole relay between ground and a suitablesupply, such as +24 volts. Upon depression of the button 16-5, thewinding 16-3 is energized to effect closure of the contacts 16-1 andapply the potential on the conductor 34 to the oscillator formed by theunijunction transistor 62 and its associated components to initiategeneration of a voltage pulse train. Other contacts of the relay operateother circuitry in the oscilloscope to effect operation in a specifiedmanner, for example, to obtain a predetermined gain of an amplifier,etc. Upon closure of the contacts "5-1, a potential is provided to thebase of a transistor 79 by way of a resistor 80 to turn on thetransistor in a saturated condition thereby shunting the voltageappearing at the upper terminal of the resistor 44 to the point 23. Thisaccomplishes the same end as grounding the emitter of thetransistor 45to turn off all DC current which could otherwise appear at thecollectors of the transistors 17 and 18. Simultaneously, however, theunijunction transistor has begun producing voltage pulsesv The pulsesappearing at the first base of the unijunction transistor 62 are shapedby the zener diode 62', the resistor 69, the capacitor 70 and theresistors 7174 and coupled to the input 30 of the operational amplifierby way of the capacitor 75.

During normal operation in particle analysis, the output of the currentgenerator is connected to the orifice and to the amplifier 11 by way ofa pair of resistors 19 and 20, modestly large resistance value, andcoupled to ground by way of a large filtering capacitor 78. The filter19, 20, 78 would not permit the current normalizing pulses to reach theorifice and effect the desired measurement. Therefore, it is necessaryto bypass the filter and means are provided for accomplishing such abypass function. A second relay winding, for example, a small reed relaywinding 16-4 is connected in parallel with the relay winding 16-3 andoperated therewith under the control of the normalize button 16-5 sothat upon depression of the normalize button 16-5, a set of contacts16-2 are operated to bypass the filter circuit and connect thecollectors of the transistor 17, 18 directly to the circuit of theorifice 1 and the amplifier 1 1. In addition, the amplifier l 1 isprovided with a feedback circuit including a resistor 76 and a capacitor77 connected in a well known bootstrap configuration to increase theinput impedance of the amplifier 11 and provide a unity gain therefor.The feedback circuit is connected between the output of the amplifier 11and a non-inverting junction between the resistors 19, 20.

Although I have described my invention by reference to a specificexemplary embodiment thereof, including specific reference to anoperating circuit realization, many changes and modifications of myinvention may become apparent to one skilled in the art withoutdeparting from the spirit and scope of my invention, and it is to beunderstood that I intend to include within the patent warranted hereonall such changes and modifications as may reasonably and properly beincluded within the scope of my contribution to the art.

I claim:

1. Apparatus for normalizing the current delivered to a particle sensingzone which has a variable resistance characteristic aNd which has acurrent generating device coupled thereto, said apparatus comprising:

a voltage source operable to provide a precise voltage;

current generator means connected to said sensing zone and to saidvoltage source and operable to provide particle sensing current inaccordance with the voltage supplied thereto;

voltage pulse generator means connected to said current generator meansand operable to superimpose voltage pulses on the voltage supplied tosaid current generator means to effect the provision of simulatedparticle current pulses at said sensing zone; and

adjusting means for adjusting the particle sensing current and thesimulated particle current pulses simultaneously.

2. Apparatus according to claim 1, wherein said voltage pulse generatormeans includes means connected to said voltage source to receive asupply potential therefrom and operable to generate said voltage pulsesin accordance with the value of supply potential received, and whereinsaid adjusting means includes means for simultaneously varying thevoltage potentials supplied to said voltage pulse generator means andsaid current generator means.

3. Apparatus according to claim 1, wherein said voltage pulse generatormeans includes an oscillator for producing pulses and a pulse shaper forshaping the pulses of the oscillator to a Gaussian shape.

4. Apparatus according to claim 1, wherein said current generator meansincludes an input amplifier having first and second inputs, said firstinput connected to said voltage pulse generator means, an outputamplifier coupled between said input amplifier and said sensing zone,and a stability feedback circuit connected between said output amplifierand said second input of said input amplifier.

5. Apparatus according to claim 1, wherein said adjusting means includesfirst means in said voltage source for providing amplitude doublings ofvoltage ranges to said current generator means and to said voltage pulsegenerator means and second means for varying the potential within thedoublings.

6. Apparatus according to claim 1, comprising means for turning saidvoltage source on and off.

7. Apparatus according to claim 1, comprising means operable to shuntsaid voltage source and render said apparatus operable in response tosaid voltage pulses only.

8. in particle size analysis apparatus of the type which has a source ofdirect current connected to a particle sensing zone having a variableresistance characteristic and operable to modulate the direct currentand generate a pulse in response to a particle traversing the sensingzone, a pulse sensing amplifier preceded by a filter connected to thesource of direct current, means connected to the amplifier forindicating the size of the particle pulses, start-stop means includingstart and stop contacts operable to initiate and terminate operation ofthe particle analysis apparatus, the improvement therein comprising:

said source of direct current including an adjustable voltage sourceoperable to provide selected potentials and a direct current generatorconnected to said voltage source and operable to provide direct currentin accordance with a selected potential; and

a voltage pulse source connected between said adjustable voltage sourceand said direct current generator for controlling said direct currentgenerator to simulate particle pulses in accordance with a selectedpotential.

9. Particle size analysis apparatus, comprising:

a particle sensing zone for receiving a flow of particles and having avariable resistance characteristic, said zone including an orifice tubeand an electrolyte immersing said tube and for suspending particlestherein;

a source of direct current connected to said particle sensing zone, thedirect current being modulated by particles traversing the orifice ofsaid tube to generate particle pulses, said source of direct currentincluding an adjustable voltage source operable to provide selectedpotentials and a current generator operable to provide a direct currentin accordance with a selected potential;

display means connected to said sensing zone for indicating themagnitude of particle pulses;

particle pulse simulator means connected to said current generator andeffective to cause said current generator to produce simulated particlepulses; and means for adjusting the output of said current generator tonormalize the current delivered to said sensing zone and compensate itsvariable resistanrfe. 10. In particle size analysis apparatus of thetype wherein at a sensing zone particles are suspended in an electrolyteand caused to flow through an orifice of a tube immersed in theelectrolyte, a source of direct current has terminals in the electrolyteon each side of the orifice, and means are connected to the terminalsfor sensing modulations of the direct current as particle pulses inresponse to particles traversing the orifice and

1. Apparatus for normalizing the current delivered to a particle sensingzone which has a variable resistance characteristic and which has acurrent generating device coupled thereto, said apparatus comprising: avoltage source operable to provide a precise voltage; current generatormeans connected to said sensing zone and to said voltage source andoperable to provide particle sensing current in accordance with thevoltage supplied thereto; voltage pulse generator means connected tosaid current generator means and operable to superimpose voltage pulseson the voltage supplied to said current generator means to effect theprovision of simulated particle current pulses at said sensing zone; andadjusting means for adjusting the particle sensing current and thesimulated particle current pulses simultaneously.
 2. Apparatus accordingto claim 1, wherein said voltage pulse generator means includes meansconnected to said voltage source to receive a supply potential therefromand operable to generate said voltage pulses in accordance with thevalue of supply potential received, and wherein said adjusting meansincludes means for simultaneously varying the voltage potentialssupplied to said voltage pulse generator means and said currentgenerator means.
 3. Apparatus according to claim 1, wherein said voltagepulse generator means includes an oscillator for producing pulses and apulse shaper for shaping the pulses of the oscillator to a Gaussianshape.
 4. Apparatus according to claim 1, wherein said current generatormeans includes an input amplifier having first and second inputs, saidfirst input connected to said voltage pulse generator means, an outputamplifier coupled between said input amplifier and said sensing zone,and a stability feedback circuit connected between said output amplifierand said second input of said input amplifier.
 5. Apparatus according toclaim 1, wherein said adjusting means includes first means in saidvoltage source for providing amplitude doublings of voltage ranges tosaid current generator means and to said voltage pulse generator meansand second means for varying the potential within the doublings. 6.Apparatus according to claim 1, comprising means for turning saidvoltage source on and off.
 7. Apparatus according to claim 1, comprisingmeans operable to shunt said voltage source and render said apparatusoperable in response to said voltage pulses only.
 8. In particle sizeanalysis apparatus of the type which has a source of direct currentconnected to a particle sensing zone having a variable resistancecharacteristic and operable to modulate the direct current and generatea pulse in response to a particle traversing the sensing zone, a pulsesensing amplifier preceded by a filter connected to the source of directcurrent, means connected to the amplifier for indicating the size of theparticle pulses, start-stop means including start and stop contactsoperable to initiate and terminate operation of the particle analysisapparatus, the improvement therein comprising: said source of directcurrent including an adjustable voltage source operable to provideselected potentials and a direct current generator connected to saidvoltage source and operable to provide direct current in accordance witha selected potential; and a voltage pulse source connected between saidadjustable voltage source and said direct current generator forcontrolling said direct current generator to simulate particle pulses inaccordance with a selected potential.
 9. Particle size analysisapparatus, comprising: a particle sensing zone for receiving a flow ofparticles and having a variable resistance characteristic, said zoneincluding an orifice tube and an electrolyte immersing said tube and forsuspending particles therein; a source of direct current connected tosaid particle sensing zone, the direct current being modulated byparticles traversing the orifice of said tube to generate particlepulses, said source of direct current including an adjustable voltagesource operable to provide selected potentials and a current generatoroperable to provide a direct current in accordance with a selectedpotential; display means connected to said sensing zone for indicatingthe magnitude of particle pulses; particle pulse simulator meansconnected to said current generator and effective to cause said currentgenerator to produce simulated particle pulses; and means for adjustingthe output of said current generator to normalize the current deliveredto said sensing zone and compensate its variable resistance.
 10. Inparticle size analysis apparatus of the type wherein at a sensing zoneparticles are suspended in an electrolyte and caused to flow through anorifice of a tube immersed in the electrolyte, a source of directcurrent has terminals in the electrolyte on each side of the orifice,and means are connected to the terminals for sensing modulations of thedirect current as particle pulses in response to particles traversingthe orifice and displaying particle size information in accordance withthe size of the particle pulses, the improvement therein comprising:means connected to said source of direct current and operable to causesaid source to provide simulated particle pulses at its terminals; andadjustable means for normalizing the output of said source of directcurrent to compensate for resistance variations effected by the orifice,the electrolyte and the counter emf contact potentials of the terminals.